1,720,965 research outputs found
VALIDAZIONE MEDIANTE APPROCCIO MAPOD DELLA TECNICA “SONDA ROTANTE” APPLICATA AL CONTROLLO UT DI ASSILI PIENI
No full textIl controllo in esercizio di assili pieni riveste un aspetto sempre più critico per il trasporto ferroviario in Europa, alla ricerca di soglie di sicurezza superiori ma, allo stesso tempo, costi di manutenzione contenuti. Le più diffuse tecniche di controllo prevedono l’ispezione dalla superficie dell’assile e richiedono, quindi, l’asportazione della vernice protettiva e, in molti casi, lo smontaggio di componenti come, ad esempio, i cuscinetti. Una tecnica alternativa, in uso in Italia sin dagli anni ‘70, prevede di effettuare l’intero controllo della superficie esterna dell’assile attraverso una scansione dall’estremità dello stesso, senza necessità di rimuovere componenti o vernici protettive. Tale tecnica è di natura ultrasonora e si basa su una sonda rotante capace di ispezionare le zone critiche degli assili mediante trasduttori diversamente angolati ed emittenti onde longitudinali tipicamente a 4 MHz. L’integrità strutturale viene quindi garantita, insieme agli altri fattori in gioco, dalla derivazione affidabile delle curve “Probability of Detection” della sonda rotante.
Nella presente memoria viene esplorata, in particolare, l’applicazione al trasporto merci. Vista l’onerosità della derivazione puramente sperimentale della curva POD, la presente ricerca descrive un approccio “Model-Assisted Probability of Detection”, dove si affiancano, alle risultanze sperimentali provenienti da difetti artificiali realizzati in assili, i risultati di opportune simulazioni numeriche eseguite col software dedicato CIVAnde. La combinazione di risultati numerici e sperimentali, rispetto all’approccio puramente sperimentale tradizionale, permette una maggiore flessibilità sui casi analizzati, l’opportunità di ottimizzare le procedure di controllo ed un risparmio significativo nella determinazione dell’affidabilità dei controlli tramite le curve “Probability of Detection”
Validation of the Rotating UT Probe for In-Service Inspections of Freight Solid Axles by Means of the MAPOD Approach
No full textThe present scenario of freight applications in Europe is characterized by a trend towards rolling stock renewal and a new approach to periodical inspection and maintenance; this is a consequence of some evidences coming from service and showing potential inconsistency in general safety levels. A specific NDT technique, based on more than 40 years of service experience on passenger wheel-sets inspection, is here proposed for periodical NDT on freight wheel-sets. The ultrasonic testing technique is based on a rotating probe-holder to be applied at the ends of the mounted axle and able to inspect its critical sections with different angled probes. To inspect such critical sections such as press fits and geometrical transitions, the testing device is characterized by a set of ultrasonic transducers emitting longitudinal waves at a typical nominal frequency of 4 MHz and at different angles into the axle. The structural integrity of the wheel-set is then based, together with other factors, on the determination of the Probability of Detection (POD) curve of the rotating probe. An experimental campaign to derive the POD curve can be costly and time consuming, especially considering the determination of its confidence range. For this reason, in the present research, a Model-Assisted POD (MAPOD) approach was considered. Initially, different sets of artificial defects, obtained by EDM, were introduced on the external surface of three solid axles made of A1N steel grade. All the defects were then inspected by a rotating probe, with the aim to derive their response, considering 4 MHz and 2.25 MHz transducers. The effect of the different time of flight characterizing the defects located at the different sections of the axles was, instead, determined by suitable numerical simulations carried out using the CIVAnde dedicated software package. Combining experimental and numerical results allowed to derive the MAPOD curve for the considered rotating probe with less effort with respect to the traditional experimental approach and to optimize the set-up of the rotating probe itself. As a result, the MAPOD curves of the rotating probe device are compared to the POD of standard UT inspection from axle end and a simulation of the reliability of inspection intervals is presented
ULTRASONIC CHARACTERIZATION OF RESIDUAL STRESSES IN RAILWAY WHEELS
No full textConsidering monoblock railway wheels for freight applications, a compressional state is required in order to guarantee the structural integrity during service. Unfortunately, adverse thermo-mechanical stresses, caused by hard braking, can reverse the beneficial compressive hoop stress to a tensile one, easing the triggering of fatigue phenomena. For this reason, periodical in-service stress measurements are required.
The present paper, carried out in the frame of the EMATWHEEL Italian project co-funded by Regione Lombardia and MIUR, aims to define qualitatively and quantitatively, through non-contact ultrasonic EMAT transducers, the residual stress of railway wheels. In particular, it is here derived via the acoustical birefringence effect and elaborated by a more refined approach, with respect to the usual one. Tests are firstly performed onto specimens, cut from railway wheels and subjected to a known biaxial stress state, and then to wheels coming from production and made of ER7 and ER8 steels
PROBABILITÀ DI RILEVAZIONE DI DIFETTI IN ASSILI CAVI MEDIANTE BOROSONDA
No full textGli assili cavi, nati negli anni ’70 per applicazioni ferroviarie ad alta velocità su treni tiltanti, trovano oggigiorno sempre maggiori applicazioni anche su veicoli metropolitani ed intercity. Infatti, al vantaggio dato dalla riduzione delle masse non sospese che ha guida inizialmente lo sviluppo di questi componenti, si somma quello di un controllo dalla superficie interna che ora può offrire una sensibilità ed affidabilità superiori rispetto alle tecniche convenzionali. Il controllo dell’assile cavo viene eseguito, infatti, mediante metodo UT e tecnica detta “borosonda” che consta in una sonda multi-cristallo roto-traslante all’interno del foro bareno. Una serie di trasduttori, diversamente angolati, ispeziona l’intera superficie esterna dell’assile allo scopo di individuare cricche e difetti. L’integrità strutturale viene quindi garantita, insieme agli altri fattori in gioco, dalla derivazione affidabile delle curve “Probability of Detection” della borosonda che costituiscono un dato di input fondamentale nella definizione della frequenza di controllo (intervalli di ispezione). Allo scopo di derivare tali curve POD, basandosi su un campione statistico significativo di misure sperimentali, si è costruito un lotto unico nel suo genere di assili contenenti cricche a fatica naturali; dieci assili cavi sono stati infatti sollecitati a fatica in modo da generare due cricche di fatica in ognuno di essi. In aggiunta, su un assile sono stati introdotti, mediante elettroerosione, difetti artificiali la cui morfologia ricalca quella dei danneggiamenti tipici da esercizio. In questo modo è stato definito un insieme di 28 difetti caratterizzato da un’escursione di profondità da 0.4 a 12 mm. L’ispezione con metodo UT di tale insieme di difetti, avvalendosi della tecnica boro sonica, ha permesso il confronto di differenti procedure d’esame attraverso il confronto delle curve POD caratteristiche. La fase finale dello studio è consistita nell’ottimizzazione della procedura d’esame stessa grazie a simulazioni numeriche dedicate
Probability of Detection of Ultrasonic In-Service Inspection of Hollow Axles
No full textHollow axles, widely employed in High Speed and Very High Speed railways applications, are typically inspected by the ultrasonic testing method applied using a suitable boreprobe roto-translating along the longitudinal bore. Different ultrasonic transducers, emitting shear waves at 2-4 MHz and at different refraction angles in steel, are used to inspect the whole external surface of the axle in order to individuate cracks and defects at press-fit seats and at the body, especially along geometrical transitions. The structural integrity of the wheel-set is then based, together with other factors, on the efficient and reliable determination of the Probability of Detection (POD) curve of the boreprobe. For a given probes configuration, the POD curve is function of UT equipment, its calibration and its inspection parameters, therefore the same UT equipment could develop different POD curves as a function of the operating procedure normally defined by wheel-set manufacturers or end users. It is then important to split the absolute performances of the UT equipment by the different performances obtainable with different calibration and operation procedures. In order to derive such curves, a statistically representative sample of natural defects is needed. For this purpose, ten full-scale hollow axles, made of a quenched and tempered alloyed railway steel, were fatigue tested using a dedicated bench in order to develop two natural fatigue cracks in each of them. Different artificial defects were also introduced on the external surface on one of the axles by EDM. A total set of 28 semi-circular defects (17 natural and 11 artificial) was then obtained in a range of depths from 0.4 mm to 12 mm. Such a set allowed the experimental derivation of the POD curve of the boreprobe following the “signal response” approach. A general analysis of different calibration method leads to a comparison between different POD obtainable with the same UT inspecting device. A final part of the research consisted in the optimization of the boreprobe itself, by means of suitable numerical simulations, in terms of frequency, dimension of the transducer, angle of refraction and location of the defect/crack
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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